This invention pertains to electrical circuits for driving ultrasonic transducers, and more particularly to controllable oscillator circuits for energizing ultrasonic transducers at their resonant frquency and for following any changes therein.
Ultrasonic devices have a wide variety of uses in the form of hand held tools for drilling, cleaning, etc., surfaces such as, for example, dental ultrasonic scalers. The hand tool generally includes a transducer for driving a vibratory tip and a circuit for energizing the transducer. The transducer can be a magnetostrictive type including an energizing coil, or a piezoelectric type to which electrical ultransonic signals are directly applied. Since the tool is handheld, it is important that the tool, when energized, remains at a comfortable temperature, particularly when used over extended periods of time. This is particularly true in the case of the dental ultrasonic scalers due to the exacting nature of the work involved.
A coolant, such as water, is generally circulated through the hand tool to maintain the temperature of the hand tool comfortable. In the case of dental ultrasonic scalers, the water is projected out from the end of the hand tool and along the tip to provide a flushing action to enhance cleaning. However, care must be taken so that the flow of water into the patient's mouth is not greater than the evacuation capacity of the dental system. If the water flow is greater than the capacity of the evacuation system, the dentist, or his assistant, is required to periodically shut off the tool and allow the water to be drawn off. This is particularly true in the case of older type dental chairs wherein the evacuation capacity may be insufficient. This requirement for periodically stopping the cleaning procedure is annoying to the dentist, is also inefficient requiring additional time for the cleaning procedure, and perhaps uncomfortable to the patient.
The combination of the transducer (coil and magnetostrictive element) and tool tip generally has a natural resonant frequency characteristic. the resonant frequency characteristic generally has a bell shape. A low "Q" curve exhibits a wider frequency range with lower amplitude than a high Q curve. The transducer is driven within the resonant frequency characteristic. The higher the Q of the transducer the greater the excursion for a given input power level and therefore greater efficiency. Hence, it is highly desirable to use as high a Q transducer as possible and drive the tool within the resonant frequency band to assure high efficiency of operation and less power dissipation. In the case of a coil driven magnetostrictive unit, it is desirable to reduce the resistance of the coil as low as possible to increase the Q of the transducer and to also reduce the power dissipated within the coil.
The resonant frequency characteristic of the combined transducer and tip varies for a variety of reasons. For example, loading of the tool tip and temperature variations tend to vary its resonant frequency characteristic both amplitude and frequency wise. In addition, depending upon the type of operation to be performed, tool tip changes may be required or desired. The hand tool often includes several different types of tips that can be interchanged to provide the desired operating function. Any change in the tip structure will also change the resonant frequency characteristic of the combination.
In the prior art, the transducer and tip combination were designed as a compromise so that the transducer can be driven by an oscillator without resulting in a lockout condition, i.e., insufficient power transfer load to assure resonance. A U.S. Pat. No. 3,629,726 entitled "Oscillator and Oscillator Controlled Circuit", issued on Dec. 21, 1971, to Gabriel Popescu, disclosed an oscillator circuit including current and voltage feedback circuits that functioned to allow the band pass of the transducer and tip combination to be narrowed from that previously used, by controlling the frequency of an oscillator to the resonant frequency of the transducer. Although the circuit disclosed in the cited patent did provide improved power transfer and allowed the use of higher Q circuits, it still did not provide the degree of efficiency desired to assure that the temperature of the hand tool would be maintained within the comfortable range, nor did it provide for a low level of water flow so that the tool can be used continuously even with the older type dental chairs. Furthermore, in the arrangement disclosed, high voltage and high currents levels are required, further adding to the power level requirements and to the cost of the control devices needed to assure long life operation under such conditions.
It therefore would be highly advantageous if a higher Q transducer could be efficiently driven by a low voltage level generator circuit and that could accurately follow small variations in the resonant frequency characteristic due to loading effects, temperature changes, tip changes, and the like, and maintain the operation of the transducer at higher efficiency to reduce heating effects in the hand tool.
It is therefore an object of this invention to provide a new and improved ultrasonic signal generator for use with higher Q ultrasonic transducers.
It is also another object of this invention to provide a new and improved ultrasonic signal generator for providing for the more efficient transfer of energy to an ultrasonic transducer and thereby reducing heating of the transducer.
It is still a further object of this invention to provide a new and improved ultrasonic signal generator that accurately follows small changes in the resonant frequency characteristic of a connected transducer due to loading affects, temperature changes, tool changes, and the like.
It is still a further object of this invention to provide a new and improved ultrasonic lower voltage level signal generator that accurately follows small changes in the resonant frequency characteristic of a connected transducer due to loading affects, temperature changes, tool changes and the like.